The present invention relates to a method and apparatus for generating a phase-correlation which is essential as a resource in such quantum information science as a quantum computer, quantum communication, quantum cryptography and others.
It is often the case that a laser field generated by an optical laser is described as an electric field that a phase component in a quantum state (pure state) called a coherent state is determinate.
However, according to laser theory properly based on a quantum theory, it is clearly shown that a steady-state laser field is in a coherent state (mixed state) that a phase component is completely indeterminate. As a result, it can be understood that a phase of the laser field proceeds to the steady state and becomes diffused with time irrespective of an initial state of that phase.
In a prior art, one laser as a reference is set, and synchronization of the entire phase is achieved by continuously supplying a laser field to each part from this laser in order to eliminate the indeterminateness of the phase of the laser field in the mixed state.
Specifically, laser beams in the mixed state are divided by using a half beam splitter and supplied to each part. At this time, it can be understood from a simple calculation that quantum states of respective output ports of the half beam splitter have a phase-correlation with respect to each other. It is well known from experience that such a phase-correlation is strong relative to a decoherence due to the environment.
Such a phase-correlation is useful for a communication protocol called quantum teleportation, for example (C. H. Bennette, et al., Phys. Rev Lett. 70, 1895 (1993).).
Quantum teleportation is currently verified by an experiment system mainly using photons. It is typified as a substantive experiment (A. Furusawa et al., Science 282, 706 (1998).) of a protocol called an infinite dimension quantum teleportation (S. L. Braunstein and H. J. Kimble, Phys. Rev. Lett. 80, 869 (1998).). In this substantive experiment, an original protocol that a measured value takes a discrete value is expanded in such a manner that a measured value becomes a continuous quantity in the light of readily enabling the application of a laser beam source.
On the other hand, in relation to this substantive experiment, it is revealed that a transmission side and a reception side must be physically connected with each other in a light path which does not exist in the original protocol in order to eliminate the phase indefiniteness that the laser field essentially has and maintain a phase-correlation between the transmission side and the reception side (T. Rudolph and B. C. Sanders, Phys. Rev. Lett. 87, 077903 (2001)). In order to realize the infinite dimension quantum teleportation which does not have such an extra light path, according to investigation conducted by the present inventors, a scheme that two or more independent laser beam sources have a phase-correlation is required.